This invention relates to a process for preparing .beta.-bourbonene having a peculiar fragrance which is used as raw material of perfume or for other use, by using, essential oil of Solidago altissima L., ylang-ylang oil or other essential oil containing germacrene D.
.beta.-Bourbonene has attracted attention as a useful raw material of perfume owing to its peculiar fragrance. However, no simple process for preparing .beta.-bourbonene in a large scale with a high yield was developed.
A prior synthetic process for obtaining .beta.-bourbonene by using cyclopentanone as the starting material was proposed [J. Am. Chem. Soc., Vol. 90, 6171 (1968)]. Another process by utilizing photochemical reaction of germacrene D was also proposed [Tetrahedron Letters, No. 27, 2263 (1969)]. But the former process using cyclopentanone as the starting material requires many operations and the yield of .beta.-bourbonene is less than 3%. Moreover, the process has such a serious demerit that it is unpractical because of the necessity of using special chemicals. In the latter process utilizing the photochemical reaction of germacrene D, .beta.-bourbonene is generally obtained by the isolation of germacrene D from essential oil of Pittosprum Tobira and its conversion to .beta.-bourbonene by irradiation with light. But this process has the following unfavourable problems: the procurement of the raw material plant is not easy, processes of isolation and refining of germacrene D are complicated, germacrene D is generally unstable, etc.
A method of manufacturing germacrene D from the essential oil of Solidago altissima L., which is easily available, has also been reported [J. Agr. Chem. Soc., Japan, Vol. 49, 245 (1975)]. However, by this method, germacrene D can only be obtained according to the following complicated operations: The essential oil of Solidago altissima L. is treated by (I) weak alkali, (II) strong alkali, (III) silica gel chromatography, (IV) fractional distillation under reduced pressure, (V) column chromatography on alumina impregrated with silver nitrate, (VI) retreatment under the same condition as of (V). Moreover, this method has the following problems: the yield of germacrene D is as low as about 3%, there is a fear that germacrene D is decomposed during the column chromatography, thermal isomerization of germacrene D is apt to take place during the fractional distillation, and adoption of the chromatography makes the mass treatment of the essential oil inconvenient. For the above mentioned reason, in the application of the process for preparing .beta.-bourbonene by means of light irradiation of germacrene D isolated from the essential oil of Solidago altissima L., the yield of .beta.-bourbonene becomes extremely low and the actual adoption of this process has been difficult.
Another method of manufacturing germacrene D from ylang-ylang oil is reported [Recherches, Vol. 19, 269 (1974)]. According to this method, silver nitrate solution of about 50% in concentration is added to ylang-ylang oil to form a double salt of germacrene D and silver nitrate, and germacrene D of high purity is obtained. This method has the following problems for its application: one mole of expensive silver nitrate is required for the refining one mole of germacrene D, operations are troublesome because above-mentioned treatment should be done in a dark room and because treatment with concentrated aqueous ammonia is required to get free germacrene D from the double salt after the ethanol washing of the double salt.